The Chinese tallow tree (Sapium sebiferum) was introduced into the southeastern United States in late 1800s and has rapidly naturalized throughout the region's coastal ecosystems. Because tallow forms monotypic woodlands, we hypothesized that allelopathic interference is a mechanism by which tallow maintains and expands its presence. Laboratory experiments were performed using black willow (Salix nigra), baldcypress (Taxodium distichum) and tallow, as test species, to examine the hypothesis that aqueous tallow extracts inhibit seed germination and seedling root growth, shoot growth and mass. Extracts were prepared from tallow litter, woodland soil from under tallow trees and fresh tallow leaves. Samples were collected during October, January, April and July to determine any seasonal differences in allelopathic potential. Germination, root and shoot lengths and seedling mass differed (P < 0.05) among treatments for willow, baldcypress and tallow. Black willow germination did not vary (P > 0.05) among control and April litter or April soil treatments; the treatments designed to mimic conditions when black willow naturally germinates. Although germination was lower (P < 0.05) in April fresh leaf and July litter, soil and leaf treatments than control treatments, black willow does not germinate during July nor do black willow seeds contact fresh tallow leaves during April. Germination rates for baldcypress and tallow during April and July litter and soil treatments were higher (P < 0.05) than control treatments. Similarly, baldcypress seedling root length, shoot length and mass were the same or higher than control treatments (P < 0.05). Tallow seedling root length, shoot length and mass were lower (P < 0.05) in controls versus April and July litter and soil treatments. Our experiment questions the validity of allelopathic interference as a mechanism enhancing tallow invasion or maintaining woodlands once established. Because of its enhanced germination and seedling growth when exposed to its own experimental treatments, tallow may in fact be perpetuating its own woodland(s) by self facilitation, rather than inhibiting other plant survival by allelopathic interference.
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